1. Field of the Invention
This invention relates in general to hydraulic elevator systems, and more particularly, to a hydraulic elevator system with a hydraulic oil reservoir recirculation path which thermostatically interacts with a local microprocessor elevator controller.
2. Description of the Prior Art
Microprocessors may be pre-programmed to perform various functions in the operational control or management of a hydraulic elevator system. Conventional hardware arrangements for elevator configurations have been known to benefit from these state-of-the-art solid-state controllers, but newly defined tasks involving uniquely reconfigured hardware arrangements have yet to emerge. Remotely present is the likelihood of failure of components such as thermostats used with the present day elevator control apparatus although it has been minimized by testing procedures, statistical use extensions, and appropriate service inspections with periodic replacements.
There is a historical recognition in accident prevention that many types of failure modes can be prevented. The codification of same and the need to meet the elevator safety code requirements is given primary recognition in the implementation of elevator car running functions, door control functions and car call and corridor call functions which are of colossal concern. In the prior art of automatic temperature control for the heating of the hydraulic oil used to drive a hydraulic elevator jack, the job of heating the oil to a minimum operating temperature has been done by a heater or heating coil in the area surrounding the oil reservoir. A thermostat or thermocouple has been used to sense the temperature and to initiate shutting off the associated power transformer for the heater when the temperature of the oil is near the standard. An overtemperature condition of the oil must be avoided since this condition presents a safety hazard and will result in shutting down the hydraulic elevator system and may damage the pump motor windings which is directly affected by it.
With the introduction of microprocessor based elevator controllers, the compliance with the safety codes has gradually shifted to implementation of the codes by programs which can cycle through an over temperature safety checking function in a very short time interval. This improvement does not, however, solve the problem inherent in operating the heater which still presents overshoots of temperature since the oil is not uniformly without temperature gradients, and the art of thermostatic control requires external apparatus and feedback circuits in order to deal with this problem.
One of the principal problems with a hydraulic elevator system of the type described above is that bulky heating coils connected with special power transformers and switching circuits are required in order to get the job done of heating the oil up to the operating temperature, and these components require space in the machine room where space is at a premium. They give off excessive heat to the adjacent area which heat must usually be dissipated, and they are costly to install and to replace if they fail.
Yet another problem present with external heaters is that the thermostat may fail and give a false signal for either coil oil or maximum temperature limits, and the heater circuit may not monitor this false signal quickly enough to avoid the damage to the pump windings or to detect when the shut down is necessary. Another problem in need of a solution is at the edge of the standard temperature where a flickering cold oil thermostat could damage the heater circuit apparatus by causing it to cycle in and out of service at a damaging frequency.